1. Field of the Invention
The invention relates to a light emitting device and a method for manufacturing the same.
2. Description of the Related Art
Inorganic electroluminescent (EL) devices, as self-emitting light sources, have been expected for usage as new display devices or the like with no need of separate illumination sources.
There are two types, “dispersion type” and “thin film type”, for conventional EL devices and many of them emit light by an AC operation.
With respect to conventional dispersion type and thin film type EL devices, inorganic EL devices disclosed in Japanese Unexamined Patent Publication No. 2007-265986 and “Latest Trend of Inorganic EL Developments, Material Properties and Fabrication Techniques/Application Expansion, 1st edition, Information Organization, Mar. 27 (2007)” have been developed by using inorganic compounds.
The conventional dispersion type EL devices emit light by phosphor particles with applying an AC voltage to devices containing phosphor particles (e.g. ZnS: Cu, Cl, etc.) shut out of an electric circuit between electrodes. It is supposed that a particle diameter of the phosphor particles is optimum to be about 10 μm and it has been known that if the diameter becomes smaller than 2 to 3 μm, emission luminance is considerably lowered. In this connection, the dispersion type EL devices are supposed to emit light due to donor-acceptor pair recombination.
Further, the conventional thin film type EL devices emit light by light emitting layer with applying an AC voltage to devices having a light emitting layer (e.g. ZnS: Mn; base material ZnS doped with emission center Mn) of a phosphor sandwiched by insulating layers between electrodes. In this connection, the thin film type EL devices are supposed to emit light due to collision excitation of an emission center by hot electrons running in a base material.
On the other hand, techniques of producing a light emitting device on a semiconductor substrate, particularly a silicon substrate, have actively been developed. Since a CMOS circuit, which is an information processor and a storage unit or the like, is manufactured by using a semiconductor including mainly silicon as a base, if a light emitting device could be manufactured simultaneously with other functional devices such as a transistor or the like on a substrate of silicon, etc., the light emitting device could be formed together with the information processor and storage unit on a single substrate. Accordingly, it is highly expected that an inter-chip communication by light and optical computing techniques are made possible and it leads to further developments of digital electronic appliances.
For example, it is known that electroluminescence having a peak at about 650 nm is confirmed by forming fine particles of silicon in a nano-meter order in a silicon nitride film (an insulator) on a silicon substrate and applying a voltage to the silicon nitride film [See Japanese Unexamined Patent Publication No. Hei 11 (1999)-310776].
In addition, with respect to a conventional light emitting device containing fine particles in an insulating film, it is necessary to apply a high voltage between both sides of the insulating film and thus apply an intense electric field of about 7 MV/cm to the insulating film. Accordingly, electrons of electrodes are supplied to a conduction band of the insulating film by FN (Fowler-Nordheim) tunneling and after being accelerated in the electric field to have sufficient kinetic energy, the electrons collide against the fine particles. The collided electrons excite electrons of the fine particles and the excited electrons are supposed to emit light.
However, there is a problem that the conventional light emitting device in which fine particles are formed in an insulating film unevenly emits light.